Plants are exposed to a wide variety of environmental stress, such as high temperature and dehydration.
The dehydration stress responsive element (DRE) is a sequence originally verified by promoter analysis of RD29A as a water stress inducible gene in the genome of Arabidopsis thaliana. The DRE binding protein (DREB) is a transcription factor isolated as a protein that binds to DRE. Among DREBs, DREB2A is an APETALA2/ethylene responsive element-binding factor-type (AP2/ERF-type) transcription factor, which was isolated as a DRE-recognizing protein (Sato, H., Plant Cell 26: 4954-4973, 2014). Since DREB2A is strongly induced by dehydration stress and high salt concentration stress, DREB2A is considered to be a transcription factor that functions under dehydration stress and high temperature stress conditions (Sakuma, Y. et al., Proc. Natl. Acad. Sci., U.S.A., 103: 18822-18827, 2006). DREB2A activity is post-translationally regulated, and a region of 30 amino acids adjacent to the AP2/ERF DNA binding domain is considered to play a key role in post-translational protein regulation. DREB2A CA, which is derived from DREB2A via deletion of such region, still has the activity, and DREB2A CA-overexpressing Arabidopsis thaliana exhibits a dwarf phenotype. Thus, DREB2A CA has been verified to improve dehydration stress resistance to a significant extent (Sakuma. Y. et al., Plant Cell 18: 1292-1309, 2006).
Sato, H., Plant Cell 26: 4954-4973, 2014 and WO 2013/111755 each report the mechanism of a plant in which DREB2A induces a target gene when the plant receives high temperature stress. Sato, H., Plant Cell 26: 4954-4973, 2014 and WO 2013/111755 each propose the mechanism, such that a protein DPB3-1 (or NF-YC 10) interacts with DREB2A and induction of high temperature stress resistance gene expression is promoted by DREB2A. In addition, Sato. H., Plant Cell 26: 4954-4973, 2014 demonstrates that the DPB3-1/DREB2A interaction would not affect the expression of dehydration stress-inducible genes.
JP Patent No. 4219711 describes that a rooting rate is improved and the life of cut flower is prolonged in a transgenic plant into which the DREB2A gene is introduced.
Allantoin (5-ureidohydantoin) is an intermediate product that is an intermediate product generated during the process of degradation of nucleic acid bases (purine bases). In plant bodies, allantoin is generated from 5-hydroxyisouric acid with the aid of allantoin synthase (AS) and then degraded into allantoic acid with the aid of allantoinase (ALN).
Watanabe, S. et al., Plant Cell Environ., 37: 1022-1036, 2014 reports that an aln-1 mutant of Arabidopsis thaliana, in which the ALN gene was deleted, accumulated allantoin in the plant body and had higher dehydration/dry stress resistance than that of a wild-type plant. It also discloses that production of abscisic acid was promoted when allantoin was administered to wild-type Arabidopsis thaliana. Accordingly, it is considered that the promoted production of abscisic acid is related to the improvement of dehydration/dry stress resistance by allantoin.
Watanabe et al., Abstracts of the 55th Annual Meeting of the Japanese Society of Plant Physiologists. PF044 (0461), 2014 reports that plant growth was promoted via application of allantoin to Arabidopsis thaliana. 
US 2010/0333237 discloses a method of protecting a plant from stress via application of ureide, such as allantoin, to a plant. US 2010/0333237 also discloses that a plant is damaged by oxidative stress upon environmental disturbance, such as droughts or coldness and that a plant is protected from damage since the scavenger pathway is promoted by high ureide concentration. However, dehydration stress resistance is different from high temperature stress resistance in a plant.
The mechanism of developing the dehydration stress resistance is different from that of the high temperature stress resistance. According to Sato, H., Plant Cell 26: 4954-4973, 2014, for example, the DPB3-1/DREB2A interaction is necessary in order to induce high temperature stress resistance by DREB2A as described above.
However, Sato, H., Plant Cell 26: 4954-4973, 2014 describes that such interaction is not correlated with induction of dehydration stress resistance. As is apparent from the foregoing description, a certain component that is capable of improving resistance to stress other than high temperature stress, such as dehydration stress, is not always capable of improving high temperature stress resistance. On the basis of the finding of the past such that allantoin has activity of improving resistance to several types of stress other than high temperature stress of a plant, accordingly, it is not possible to predict activity of allantoin concerning high temperature stress resistance.
The expression of DREB2A gene of a plant contributes to an improvement in resistance to various types of stress, such as high temperature stress resistance or dehydration stress resistance, as described above. In addition, the expression of DREB2A gene is known to exert advantageous effects such as an improved rooting rate or prolonged life of cut flower. However, a substance that is applied to a plant and capable of promoting the expression of DREB2A gene in the plant has not been reported in the past, although transgenic plants into which the DREB2A gene is introduced are described in WO 2013/111755.
The effect of allantoin on rice growth has also been reported. Kyo et al., Guangxi Agricultural Sciences, 1999, 3rd phase, pp. 122 to 124 discloses that, when rice seeds were immersed in a 300 mg/L allantoin solution for 24 hours, germinated, and then allowed to grow up to seedlings, the height of the seedlings, the number of roots, and the fresh weight of the seedlings increased as compared to a control sample being treated with water, promoting the growth of the rice.
Although Watanabe, S. et al., Plant Cell Environ., 37: 1022-1036, 2014 discloses that applying allantoin to Arabidopsis thaliana promotes production of abscisic acid, the relationship between abscisic acid and the plant growth has not been elucidated.